Fullerene Derivatives
Significant progress has been made in the development of thin-film organic electronic devices, such as photovoltaic cells, transistors, photodetectors, sensors, and other devices for commercial application. Many of these devices utilize solution-processable semiconductors based on fullerene derivatives in pure form. The most commonly used fullerene derivative is Phenyl-C61-Butyric-Acid-Methyl-Ester ([60]PCBM) (Scharber et al., Advanced Materials (18) 789-794), which is classified as a methanofullerene. Another methanofullerene derivative is Thiophenyl-C61-Butyric-Acid-Methyl-Ester ([60]ThCBM). Methanofullerenes possess many benefits compared to the native (un-derivatized) fullerene in organic electronics applications. One benefit is their increased processability compared to native fullerenes, while maintaining much of the desirable electronic properties of the native fullerene. The increase in processability is due, in part, to an approximately ten-fold increase in solubility in aromatic solvents. Non-methanofullerene derivatives include 2-Aza-Propano-(Cn+2N) fullerenes, also called Prato adducts ([60]Prato).

[60]PCBM is typically blended with various conducting polymers which act as the electron donor and the [60]PCBM as the electron acceptor. Solution processing is used to form thin films comprising the electron donor and electron acceptor. Although fullerene derivatives are typically blended with various conducting polymers, it is possible to construct devices using only fullerene derivatives. Fullerenes and fullerene derivatives also have been shown to exhibit ambipolar electronic behavior, transporting either holes or electrons, or both, in the same device. Typical applications of [60]PCBM use [60]PCBM having a purity of about 99% or higher. Impurities in such compositions often comprise low amounts of pure C60 fullerene, certain PCBM analogues of C60 fullerene, oxides of C60 fullerene, oxides of PCBM, and trace amounts of other fullerenes. [60]PCBM is typically synthesized using C60 fullerene as the synthetic precursor where the purity of the C60 fullerene is typically about 99% or higher with respect to oxide impurities, C60-dimer, and small amounts of other fullerenes.
[70]PCBM is an analogue of [60]PCBM that can be prepared by derivatization of C70 fullerene using a process analogous to that used to prepare [60]PCBM from C60 fullerene. [70]PCBM has been used as a semiconductor in organic electronics, particularly for polymer solar devices (Wienk et al., Angewadte Chemie, 2003, (115), 3493-3497) and transistors (Anthopoulos et al., Journal of Applied Physics, (98), 054503). Similar to [60]PCBM, [70]PCBM is typically used in purities of approximately 99%, though several isomers of [70]PCBM are present due to the asymmetry of the C70 and resulting difference in reactivity of the carbons of the C70 molecule. [70]PCBM is typically synthesized from C70 fullerene material having a purity of about 99%, and the impurities in C70 fullerene are often similar to the impurities found in C60. In comparison to [60]PCBM, [70]PCBM is somewhat more soluble in organic solvents, possibly due to the presence of multiple isomers. A related analog, [84]PCBM, has also been synthesized and tested in organic photodiodes and transistors.
Organic Electronic Devices
Organic electronic devices, such as bulk heterojunction photodiodes (Scharber et al., Advanced Materials (18) 789-794), are based on forming thin films (˜150 nm) of an electronic polymer and PCBM in the so-called donor/acceptor configuration where the polymer and PCBM phase-separate to form sub-micron size-scale domains with varying degrees of amorphous and crystalline structure. In the process of forming the heterojunction thin film, the polymer and PCBM phase-separate and precipitate from solution upon drying and/or annealing of the film. Due to influences on electron transport the size domains of the PCBM and degree of crystallinity have a strong impact on the resulting electronic behavior and performance of the device. It has been shown through experimental and theoretical analyses that the electron mobility (and resulting current-carrying capacity, a strong determiner of the energy conversion efficiency) can be described with a Gaussian disorder model (Mihailetchi et al., Advanced Functional Materials, 2003, (13), 1.), which is known also to be the case for other materials, such as pure conducting polymers. In this model, the degree of disorder (or lack of crystallinity) has a strong influence on the electron mobility which has been seen with the drop in electron mobility of PCBM compared to single crystal C60. Therefore, higher degrees of amorphous nature (i.e., lower crystallinity) lead to reductions in electron mobility and corresponding reductions in energy conversion efficiency for organic electronic devices. This effect can also be seen through the increase in energy conversion efficiency gained through annealing, which is known to give greater crystallinity (less disorder or less amorphous nature) to the film (Ma et al., Advanced Functional Materials, 2005 (15), 1617-1622).
Virtually all organic electronic devices utilize a single n-type semiconductor because certain impurities in the ppm level or even ppb level can drastically alter device performance. For example, impurities in the ppm level or even ppb level can drastically alter device performance for silicon-based electronics. The impact that impurities can have on device performance is linked in part to the disorder described in preceding paragraph. However, impurities can alter the electronics of the device for other reasons, including short-circuiting and electron trapping.
Various methanofullerene PCBM analogues have been made where solubility is increased by the addition of C4, C8, C12, or C1-6 alkane units to the ester of the PCBM (Zheng et al., Journal of Physical Chemistry B (108), 32, 2004). Despite almost identical reduction potentials and UV/VIS absorption properties of the fullerene derivatives tested, the energy conversion efficiency is quite different for these different fullerene derivatives when used to fabricate organic solar cells with the same polymer. This may be attributed to the different molecular interactions between the polymer and fullerene derivative, which affect the morphology of the composition. The result may also be attributed to decreases in electron mobility brought about by the slight increase in the different path-lengths to the C60 fullerene core that functions as the electron acceptor. The decrease in energy conversion efficiency from 2.45 to 1.46 when PCB-C8H17 is used instead of PCB-C4H9 illustrates how the energy conversion efficiency is sensitive to changes in the structure of the fullerene derivative used as the n-type semiconductor. For the purposes of comparison, PCB-C16H33 gives an energy conversion efficiency of 0.11. This shows that such changes may occur solely in response to physical changes to the film brought about by changes in the n-type semiconductor, even when n-type semiconductor compounds are used that are identical in reduction potential and light absorption spectrum.

Thin-film organic electronics device performance depends on a large set of processing and materials parameters, with a high degree of complexity in the interaction of these parameters to make up the final device morphological and electronics properties. It is often not possible to predict the effect that a change in the molecular structure of an n-type semiconductor fullerene derivative will have on the final organics electronics device performance, even knowing the reduction potential, absorption properties, and other electronic properties. This is mainly due to the fact that the impact on the physical disorder of the resulting thin-film multi-phase system is difficult to predict a priori. Likewise, a change in type and level of impurities present in a given fullerene derivative n-type semiconductor may affect the morphology and electronics properties and resulting device performance of a thin-film device in an unpredictable manner.
However, in view of the foregoing obstacles, new materials are needed for thin-film organic electronic devices. The present invention fulfills this need and has other related advantages.